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Technology in Cancer Research & Treatment
ISSN 1533-0346
Volume 3, Number 1, February (2004)
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Nanoshell-Enabled Photonics-Based
Imaging and Therapy of Cancer
www.tcrt.org
Metal nanoshells are a novel type of composite spherical nanoparticle consisting of a dielectric core covered by a thin metallic shell which is typically gold. Nanoshells possess highly
favorable optical and chemical properties for biomedical imaging and therapeutic applications. By varying the relative the dimensions of the core and the shell, the optical resonance
of these nanoparticles can be precisely and systematically varied over a broad region ranging from the near-UV to the mid-infrared. This range includes the near-infrared (NIR) wavelength region where tissue transmissivity peaks. In addition to spectral tunability, nanoshells
offer other advantages over conventional organic dyes including improved optical properties
and reduced susceptibility to chemical/thermal denaturation. Furthermore, the same conjugation protocols used to bind biomolecules to gold colloid are easily modified for nanoshells.
In this article, we first review the synthesis of gold nanoshells and illustrate how the core/shell
ratio and overall size of a nanoshell influences its scattering and absorption properties. We
then describe several examples of nanoshell-based diagnostic and therapeutic approaches
including the development of nanoshell bioconjugates for molecular imaging, the use of scattering nanoshells as contrast agents for optical coherence tomography (OCT), and the use
of absorbing nanoshells in NIR thermal therapy of tumors.
Christopher Loo, B.S.1
Alex Lin, B.S.1
Leon Hirsch, B.S.1
Min-Ho Lee, M.S.1
Jennifer Barton, Ph.D.2
Naomi Halas, Ph.D.3
Jennifer West, Ph.D.1
Rebekah Drezek, Ph.D.1,*
1Department
of Bioengineering
Rice University
P.O. Box 1892, MS-142
Houston, TX 77030
2Biomedical
Engineering Program
University of Arizona
1501 N. Campbell
P.O. Box 245084
Key words: Biophotonics, Contrast agents, Photothermal therapy, Nanotechnology.
Introduction
Tucson, Arizona 85724
3Departments
of Electrical and
Computer Engineering
There is a significant clinical need for novel methods for detection and treatment
of cancer which offer improved sensitivity, specificity, and cost-effectiveness. In
recent years, a number of groups have demonstrated that photonics-based technologies are valuable in addressing this need. Optical technologies promise high
resolution, noninvasive functional imaging of tissue at competitive costs.
However, in many cases, these technologies are limited by the inherently weak
optical signals of endogenous chromophores and the subtle spectral differences
of normal and diseased tissue. Over the past several years, there has been
increasing interest in combining emerging optical technologies with the development of novel exogenous contrast agents, designed to probe the molecular specific signatures of cancer, to improve the detection limits and clinical effectiveness of optical imaging. For instance, Sokolov et al. (1) recently demonstrated
the use of gold colloid conjugated to antibodies to the epidermal growth factor
receptor (EGFR) as scattering contrast agents for biomolecular optical imaging
of cervical cancer cells and tissue specimens. In addition, optical imaging applications of nanocrystal bioconjugates have been described by multiple groups
including Bruchez et al. (2), Chan and Nie (3), and Akerman et al. (4). More
Rice University
P.O. Box 1892, MS-366
Houston, TX 77030
* Corresponding Author:
Rebekah Drezek, Ph.D.
Email: [email protected]
Abbreviations: Near infrared (NIR), Optical coherence tomography (OCT), Polyethylene glycol (PEG)
33
34
recently, interest has developed in the creation of nanotechnology-based platform technologies which couple molecular
specific early detection strategies with appropriate therapeutic intervention and monitoring capabilities.
Loo et al.
core gold/silica nanoshell is depicted. In this figure, the core
and shell of the nanoparticles are shown to relative scale
directly beneath their corresponding optical resonances. In
Figure 3, a plot of the core/shell ratio versus resonance wavelength for a silica core/gold shell nanoparticle is displayed
(6). The extremely agile “tunability” of the optical resonance
is a property unique to nanoshells: in no other molecular or
nanoparticle structure can the resonance of the optical
absorption properties be so systematically “designed.”
Figure 1: Visual demonstration of the tunability of metal nanoshells.
Metal nanoshells are a new type of nanoparticle composed of
a dielectric core such as silica coated with an ultrathin metallic layer, which is typically gold. Gold nanoshells possess
physical properties similar to gold colloid, in particular, a
strong optical absorption due to the collective electronic
response of the metal to light. The optical absorption of gold
colloid yields a brilliant red color which has been of considerable utility in consumer-related medical products, such as
home pregnancy tests. In contrast, the optical response of
gold nanoshells depends dramatically on the relative size of
the nanoparticle core and the thickness of the gold shell. By
varying the relative core and shell thicknesses, the color of
gold nanoshells can be varied across a broad range of the
optical spectrum that spans the visible and the near infrared
spectral regions (Figure 1) (5, 6). Gold nanoshells can be
made to either preferentially absorb or scatter light by varying the size of the particle relative to the wavelength of the
light at their optical resonance. In Figure 2, a Mie scattering
plot of the nanoshell plasmon resonance wavelength shift as
a function of nanoshell composition for the case of a 60 nm
Figure 3: Core/shell ratio as a function of resonance wavelength for gold/
silica nanoshells.
Halas and colleagues have completed a comprehensive investigation of the optical properties of metal nanoshells (7).
Quantitative agreement between Mie scattering theory and
the experimentally observed optical resonant properties has
been achieved. Based on this success, it is now possible to
predictively design gold nanoshells with the desired optical
resonant properties and fabricate the nanoshell with the
dimensions and nanoscale tolerances necessary to achieve
these properties (6). The synthetic protocol developed for the
fabrication of gold nanoshells is very simple in concept:
I.
II.
III.
Figure 2: Optical resonances of gold shell-silica core nanoshells as a function of their core/shell ratio. Respective spectra correspond to the nanoparticles depicted beneath.
grow or obtain silica nanoparticles dispersed in
solution,
attach very small (1-2 nm) metal “seed” colloid
to the surface of the nanoparticles via molecular linkages; these seed colloids cover the
dielectric nanoparticle surfaces with a discontinuous metal colloid layer,
grow additional metal onto the “seed” metal
colloid adsorbates via chemical reduction in
solution.
This approach has been successfully used to grow both gold
and silver metallic shells onto silica nanoparticles. Various
stages in the growth of a gold metallic shell onto a function-
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
Nanoshell Imaging
35
alized silica nanoparticle are shown in Figure 4. Figure 5
shows the optical signature of shell coalescence and growth
for two different nanoshell core diameters.
Figure 4: Transmission electron microscope images of gold/silica
nanoshells during shell growth.
Figure 5: (a) Growth of gold shell on 120 nm diameter silica nanoparticle.
The lower spectral curves follow the evolution of the optical absorption as
coalescence of the gold layer progresses. Once the shell is complete, the
peak absorbance is shifted to shorter wavelengths. Corresponding theoretical peaks are plotted with dashed lines. (b) Growth of gold shell on 340 nm
diameter silica nanoparticles. Here the peak shifts are more pronounced
with only the shoulder of the middle curve visible in our instrument range.
Based on the core/shell ratios that can be achieved with this
protocol, gold nanoshells with optical resonances extending
from the visible region to approximately 3 µm in the infrared
can currently be fabricated. This spectral region includes the
800-1300 nm “water window” of the near infrared, a region
of high physiological transmissivity which has been demonstrated as the spectral region best suited for optical bioimaging and biosensing applications. The optical properties of
gold nanoshells, when coupled with their biocompatibility
and their ease of bioconjugation, render these nanoparticles
highly suitable for targeted bioimaging and therapeutic
applications. By controlling the physical parameters of the
nanoshells, it is possible to engineer nanoshells which primarily scatter light as would be desired for many imaging
applications, or alternatively, to design nanoshells which are
strong absorbers permitting photothermal-based therapy
applications. The tailoring of scattering and absorption
cross-sections is demonstrated in Figure 6 which shows sample spectra for two nanoshell configurations, one designed to
scatter light and the other to preferentially absorb light.
Because the metal layer of gold nanoshells is grown using
the same chemical reaction as gold colloid synthesis, the
surfaces of gold nanoshells are virtually chemically identical to the surfaces of the gold nanoparticles universally used
in bioconjugate applications. The use of gold colloid in bio-
Figure 6: Nanoshells may be designed to be predominantly scattering or
absorbing by tailoring the core and shell fabrication materials. To demonstrate this concept, the predicted scattering efficiency, absorption efficiency,
and extinction are shown for two nanoshells: (A) a scattering configuration
(core radius = 40 nm; shell thickness = 20 nm) (5) and (B) an absorbing configuration (core radius = 50 nm; shell thickness = 10 nm).
logical applications began in 1971 when Faulk and Taylor
invented the immunogold staining procedure (8). Since that
time, the labeling of targeting molecules, especially proteins, with gold nanoparticles has revolutionized the visualization of cellular or tissue components by electron
microscopy. The optical and electron beam contrast qualities of gold colloid have provided excellent detection qualities for such techniques as immunoblotting, flow cytometry,
and hybridization assays. Conjugation protocols exist for
the labeling of a broad range of biomolecules with gold colloid, such as protein A, avidin, streptavidin, glucose oxidase, horseradish peroxidase, and IgG. Successful gold
nanoshell conjugation with enzymes and antiobodies has
previously been demonstrated.
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
36
Loo et al.
In this article, we present data demonstrating the potential of
nanoshells for several biomedical applications including the
use of nanoshell bioconjugates as biological labels for optical
imaging, the development of nanoshell-based scattering contrast agents for optical coherence tomography, and the use of
absorbing nanoshells for photothermal therapy of tumors.
Cell Culture
Methods and Materials
Molecular Imaging, Cytotoxicity, and Silver Staining
Gold Nanoshell Fabrication
SKBR3 cells were exposed to 8 µg/mL of bioconjugated
nanoshells for 1 hr, washed with phosphate-buffered saline, and
observed under darkfield microscopy, a form of microscopy
sensitive only to scattered light. The calcein-AM live stain
(Molecular Probes, 1 µM) was used to assess cell viability after
nanoshell targeting. A silver enhancement stain (Amersham
Pharmacia), a qualitative stain capable of detecting the presence
of gold on cell surfaces, was used to assess cellular nanoshell
binding. Cells incubated with targeted nanoshells were fixed
with 2.5% glutaraldehyde, and exposed to silver stain for 15
minutes. Silver growth was monitored under phase-contrast,
with further silver enhancement blocked by immersion in 2.5%
sodium thiosulfate. Darkfield and silver stain images were
taken with a Zeiss Axioskop 2 plus microscope equipped with a
black-white CCD camera. All images were taken at 40X magnification under the same lighting conditions.
Cores of silica nanoparticles were fabricated as described by
Stober et al. (9) in which tetraethyl orthosilicate was reduced
in NH4OH in ethanol Particles were sized with a Philips
XL30 scanning electron microscope. Polydispersity of
<10% was considered acceptable. Next, the silica surface
was aminated by reaction with aminopropyltriethoxysilane
in ethanol. Gold shells were grown using the method of Duff
et al. (10). Briefly, small gold colloid (1-3 nm) was adsorbed
onto the aminated silica nanoparticle surface. More gold
was then reduced onto these colloid nucleation sites using
potassium carbonate and HAuCl4 in the presence of
formaldehyde. Gold nanoshell formation and dimensions
were assessed with a UV-VIS spectrophotometer and scanning electron microscopy (SEM). The nanoshells used in the
darkfield scattering imaging studies described consisted of a
120 nm silica core radius with a 35 nm thick gold shell. The
nanoshells used in the OCT imaging consisted of a 100 nm
core radius and 20 nm thick shell. The nanoshells used in the
therapy application described used a 60 nm core radius and
a 10 nm thick shell which absorb light with an absorption
peak at ~815 nm. The reader is referred to (6) for a detailed
description of nanoshell synthesis procedures.
Antibody Conjugation
Ortho-pyridyl-disulfide-n-hydroxysuccinimide polyethylene glycol polymer (OPSS-PEG-NHS, MW=2000) was
used to tether antibodies onto the surfaces of gold
nanoshells. Using NaHCO3 (100 mM, pH 8.5) OPSS-PEGNHS was re-suspended to a volume equal to that of either
HER2 (specific) or IgG (non-specific) antibodies. At this
concentration, the concentration of polymer was in molar
excess to the amount of HER2 or IgG antibody used. The
reaction was allowed to proceed on ice overnight. Excess,
unbound polymer was removed by membrane dialysis
(MWCO = 10,000). PEGylated antibody (0.67 mg/mL) was
added to nanoshells (~109 nanoshells/mL) for 1 hr to facilitate targeting. Unbound antibody was removed by centrifugation at 650 G, supernatant removal, and resuspension in
potassium carbonate (2 mM). Following antibody conjugation, nanoshells surfaces were further modified with PEGthiol (MW=5000, 1 µM) to block non-specific adsorption
sites and to enhance biocompatibility.
HER2-positive SKBr3 human mammary adenocarcinoma
cells were cultured in McCoy’s 5A modified medium supplemented with 10% FBS and antibiotics. Cells were maintained at 37º C and 5% CO2.
Optical Coherence Tomograhy (OCT)
Optical coherent tomography (OCT) is a state-of-the-art imaging technique which produces high resolution (typically 10-15
µm), real-time, cross-sectional images through biological tissues. The method is often described as an optical analog to
ultrasound. OCT detects the reflections of a low coherence
light source directed into a tissue and determines at what depth
the reflections occurred. By employing a heterodyne optical
detection scheme, OCT is able to detect very faint reflections
relative to the incident power delivered to the tissue. In OCT
imaging out of focus light is strongly rejected due to the coherence gating inherent to the approach. This permits deeper
imaging using OCT than is possible using alternative methods
such as reflectance confocal microscopy where the out of
focus rejection achievable is far lower. In the OCT experiments described in this paper, a conventional OCT system
with an 830 nm superluminescent diode was used to obtain mscans of the cuvette (images with time as the x-axis and depth
as the y-axis). The axial and lateral resolution of the OCT system were 16 µm and 12 µm, respectively. Each image
required approximately 20 seconds to acquire. System parameters remained the same throughout the experiment.
In Vitro Photothermal Nanoshell Therapy
SKBr3 breast cancer cells were cultured in 24-well plates
until fully confluent. Cells were then divided into two treat-
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
Nanoshell Imaging
37
ment groups: nanoshells + NIR-laser and NIR-laser alone.
Cells exposed to nanoshells alone or cells receiving neither
nanoshells nor laser were used as controls. Nanoshells were
prepared in FBS-free medium (2 × 109 nanoshells/mL).
Cells were then irradiated under a laser emitting light at 820
nm at a power density of ~35 W/cm2 for 7 minutes with or
without nanoshells. After NIR-light exposure, cells were
replenished with FBS-containing media and were incubated
for an additional hour at 37º C. Cells were then exposed to
the calcein-AM live stain for 45 minutes in order to measure
cell viability. The calcein dye causes viable cells to fluoresce green. Fluorescence was visualized with a Zeiss
Axiovert 135 fluorescence microscope equipped with a filter
set specific for excitation and emission wavelengths at 480
and 535 nm, respectively. Membrane damage was assessed
using an aldehyde-fixable fluorescein dextran dye. Cells
were incubated for 30 min with the fluorescent dextran,
rinsed, and immediately fixed with 5% glutaraldehyde.
Photothermal destruction of cells was attributed to hyperthermia induced via nanoshell absorption of NIR light.
As Figure 7 demonstrates, these nanoshells scatter light
strongly throughout the visible and near-infrared regions.
This permits the same nanoshells to be used in light-based
microscopy studies employing silicon CCDs and in NIR tissue imaging studies using reflectance confocal microscopy
and OCT. We also fabricated nanoshells with a 100 nm
radius and 20 nm shell thickness for OCT imaging. These
nanoshells have very similar scattering and absorption spectra to the larger nanoshells; however, the scattering and
absorption cross-sections are smaller due largely to the
smaller particle size. In addition, smaller 60 nm radius
A
Results and Discussion
As an initial demonstration of the potential of nanoshells in
cancer imaging and therapy, we designed and fabricated
nanoshells suitable for both scattering and absorption-based
photonics applications. For proof-of-principle imaging studies, we fabricated nanoshells with a 120 nm radius and 35
nm shell thickness. It should be noted that nanoshells over a
broad range of sizes can be fabricated for scattering based
imaging applications. Figure 7 displays the predicted scattering and absorption spectra for these nanoshells obtained
using software extensively verifed against Mie theory which
numerically computes optical spectra for gold nanoshells.
B
C
Figure 7: Scattering and absorbing properties of nanoshells with a 120 nm
silica core radius and a 35 nm thick gold shell predicted analytically.
Scattering maximum (705-710 nm) is 2.5-times greater than absorption
maximum (570 nm) and extends into the NIR region. Nanoshell dimensions
were assessed using scanning electron microscopy (SEM). Shell thickness
was mathematically corroborated by matching experimental measurements
to scattering theory and confirmed with SEM.
Figure 8: SEM images of nanoshells used in the described studies. The top
image (A) shows the larger diameter nanoshells used in the darkfield imaging experiments. The middle image (B) shows the nanoshells used in the
OCT experiments. The bottom image (C) show the smaller diameter
nanoshells used for photothermal therapy applications. The scale bars in (A)
and (B) are 1 µm while the scale bar in (C) is 500 nm.
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
38
Loo et al.
nanoshells with a 10 nm shell were fabricated for photothermal therapy applications. Figure 8 shows SEM images of
the nanoshells fabricated at all three sizes.
As an initial demonstration of the molecular imaging potential of nanoshell bioconjugates, we imaged carcinoma cells
which overexpress HER2, a clinically significant molecular
marker of breast cancer. Under darkfield microscopy, a form
of microscopy sensitive only to scattered light, significantly
increased optical contrast due to HER2 expression was
observed in HER2-positive SKBR3 breast cancer cells targeted with HER2-labeled nanoshells compared to cells targeted by nanoshells non-specifically labelled with IgG
(Figure 9). In addition, greater silver staining intensity was
seen in cells exposed to HER2-targeted nanoshells than cells
exposed to IgG-targeted nanoshells, providing additional
evidence that the increased contrast seen under darkfield
may be specifically attributable to nanoshell targeting of the
HER2 receptor. No differences were observed under darkfield or silver stain in HER2 and IgG-targeted nanoshells
using the HER2-negative MCF7 breast cancer cell line (data
not shown). More extensive descriptions of imaging experiments using nanoshell bioconjugates are described in (11).
Although darkfield microscopy is suitable for in vitro cell
level imaging experiments, in vivo imaging applications will
require the use of appropriate scattering-based imaging technologies such as optical coherence tomography (OCT). To
assess the suitability of nanoshells for OCT applications, we
computed the scattering efficiencies of gold nanoshells (in
A
B
saline) over a range of core radii and shell thicknesses at 830
nm as shown in Figure 10. The promising scattering crosssections (approximately several times the geometric crosssections) computed for nanoshells based on physical parameters which could be readily fabricated encouraged further
experimental investigation. To provide a basis for comparison of scattering efficiencies, a 150 nm diameter polystytene
sphere in saline at 830 nm has a scattering efficiecy of 0.009;
a 300 nm polystyrene sphere has an efficiency of 0.09. As a
visual demonstration of the potential of nanoshells for OCT
imaging applications, we imaged a 1 mm pathlength cuvette
containing one of three solutions: saline, a microspherebased scattering solution, or a solution of scattering
nanoshells in water (Figure 10). The microsphere mixture
was 0.1% solids by volume of 2 µm polystyrene spheres in
saline at a concentration which provided a scattering coefficient, µs, = 16 cm-1 and an anisotropy factor, g = 0.96. The
nanoshell (100 nm radius/20 nm shell) concentration was
approximately 109/mL. Figure 10 shows OCT images of the
cuvette with saline, microspheres and nanoshells. The
images consist of one hundred scans in the same lateral location. The average grayscale value inside the cuvette walls
was calculated using the NIH Image Analysis Program. The
OCT intensity is based on a log scale where black (255) corresponds to the noise floor of -100 dB and white (0) to -40
dB. The average grayscale intensity for saline was 247 while
the average intensity within the cuvette walls containing
nanoshells the intensity was 160. Current efforts are more
carefully exploring the potential of nanoshells as contrast
agents for OCT through in vivo imaging studies of mice after
direct injection of scattering nanoshells into the
vasculature via a tail vein catheter (12).
Figure 9: Darkfield (top row) and silver stain (bottom row) images of HER2-positive
SKBR3 breast cancer cells exposed to nanoshells conjugated with either (A) HER2 (specific) or (B) IgG (non-specific) antibodies. As demonstrated here, it is possible to exploit the
optical properties of predominantly-scattering nanoshells to image overexpressed HER2 in
living cells. Similar scattering intensities were observed when comparing cells exposed to
IgG-targeted nanoshells and cells not exposed to nanoshell bioconjugates.
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
Currently, our efforts are directed towards coupling
our nanoshell-based molecular imaging technologies to some form of triggerable therapeutic intervention. Recent studies have considered a novel
approach to cancer therapy based on the use of
metal nanoshells as near-infrared (NIR) absorbers
(13). In biological tissue, tissue transmissivity is
highest in the NIR spectral range due to low inherent scattering and absorption properties within the
region. Figure 7 demonstrates that nanoshells can
be developed to highly scatter within this spectral
regions; alternatively, nanoshells may be engineered
to function as highly effective NIR absorbers as
well. As an example of the intense absorption possible using nanoshells, the coventional NIR dye
indocyanine green has an absorption cross-section
of ~10-20 m2 at ~800 nm while the cross-section of
the absorbing nanoshells described in this article is
~4 × 10-14 m2, an approximately millionfold
increase in absorption cross-section (13). By combining NIR absorbing nanoshells with an appropri-
Nanoshell Imaging
39
A
B
Figure 10: Computed scattering efficiency for nanoshells as a function of
core radius and shell thickness at 830 nm, a wavelength commonly used in
OCT imaging applications.
ate light source, it is possible to selectively induce photothermal destruction of cells and tumors treated with gold
nanoshells. Nanoshell-mediated photothermal destruction of
carcinoma cells is demonstrated in Figure 12. After laser
exposure of 35 W/cm2 for 7 minutes, all cells within the laser
spot underwent photothermal destruction as assessed using
calcein AM viability staining, an effect that was not observed
in cells exposed to either nanoshells alone or NIR light alone.
In addition, evidence of irreversible cell membrane damage
was noted via imaging of the fluorescent dextran dye (date
not shown). This dye is normally impermeable to healthy
cells. However, the dye was found in the intracellular space
of cells exposed to both NIR nanoshells and the laser but was
not observed in cells exposed to either the NIR nanoshells or
the laser alone. The calcein AM stain and the fluorescent dextran stain can be used to indicate that the cells are not viable
and that membrane damage has occurred but do not determine
the underlying cause of cell death.
In an animal study described in (13), absorbing nanoshells
(109/ml, 20-50 µl) were injected interstially (~5 mm) into
solid tumors (~1 cm) in female SCID mice. Within thirty
minutes of injection, tumor sites were exposed to NIR light
(820 nm, 4 W/cm2, 5 mm spot diameter, <6 min).
Temperatures were monitored via phase-sensitive, phasespoiled gradient-echo MRI. Magnetic resonance temperature imaging (MRTI) demonstrated that tumors reached temperatures which caused irreversible tumor damage (∆T =
37.4 ± 6.6º C ) within 4-6 minutes. Controls which were
exposed to a saline injection rather than nanoshells experienced significantly reduced average temperatures after exposure to the same NIR light levels (∆T < 10º C ). These average temperatures were obtained at a depth of ~2.5 mm below
the surface. The MRTI findings demonstrated good agree-
C
Figure 11: OCT (830 nm) images of a cuvette filled with saline (A), cuvette
containing microspheres to approximate a scattering coefficient of 16 cm-1
(B), and cuvette containing nanoshells at a concentration of ~109/ml (C).
ment with gross pathology indications of tissue damage.
Histological indications of thermal damage including coagulation, cell shrinkage, and loss of nuclear staining were noted
in nanoshell-treated tumors; no such changes were found in
Figure 12: Calcein AM staining of cells (green fluorescence indiciates cellular viability). Left: cells after exposure to laser only (no nanoshells).
Middle: cells incubated with nanoshells but not exposed to laser light.
Right: cell incubated with nanoshells after laser exposure. The dark circle
seen in the image on the right corresponds to the region of cell death caused
by exposure to laser light after incubation with nanoshells.
Technology in Cancer Research & Treatment, Volume 3, Number 1, February 2004
40
control tissue. Silver enhancement staining provided further
evidence of nanoshells in regions with thermal damage. The
intial work described here established nanoshell and laser
dosages which provided effective nanoshell-mediated photothermal therapy. Based on the parameters identified
through these intial investigations, survival studies are now
underway. Future work will also consider nanoshells conjugated to surface markers overexpressed within tumors.
Conclusions
Combining advances in biophotonics and nanotechnology
offers the opportunity to significantly impact future strategies
towards the detection and therapy of cancer. Today, cancer is
typically diagnosed many years after it has developed usually
after the discovery of either a palpable mass or based on relatively low resolution imaging of smaller but still significant
masses. In the future, it is likely that contrast agents targeted
to molecular markers of disease will routinely provide molecular information that enables characterization of disease susceptibility long before pathologic changes occur at the
anatomic level. Currently, our ability to develop molecular
contrast agents is at times constrained by limitations in our
understanding of the earliest molecular signatures of specific
cancers. Although the process of identifying appropriate targets for detection and therapy is ongoing, there is a strong
need to develop the technologies which will allow us to image
these molecular targets in vivo as they are elucidated. In this
article, we describe the optical properties and several emerging clinical applications of nanoshells, one class of nanostructures which may provide an attractive candidate for specific in
vivo imaging and therapy applications. We have reviewed our
preliminary work towards the development of nanoshell bioconjugates for molecular imaging applications and described
an important new approach to photothermal cancer therapy.
More extensive in vivo animal studies for both cancer imaging
and therapy applications are currently underway in order to
investigate both the potential and limitations of nanoshell
technologies. Additional studies are in progress to more thoroughly assess the biodistribution and biocompatibility of
nanoshells used in in vivo imaging and therapy applications.
We believe there is tremendous potential for synergy between
the rapidly developing fields of biophotonics and nanotechnology. Combining the tools of both fields – together with the
latest advances in understanding the molecular origins of cancer – may provide a fundamentally new approach to detection
and treatment of cancer, a disease responsible for over one
quarter of all deaths in the United States today.
Loo et al.
Nanotechnology (EEC-0118007), and the Department of
Defense Congressionally Directed Medical Research
Program (DAMD17-03-1-0384).
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Acknowledgements
Funding for this project was provided by the National
Science Foundation (BES 022-1544), the National Science
Foundation Center for Biological and Environmental
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